Thin film field effect transistors (TFT) used in liquid crystal display (LCD) applications typically use amorphous silicon (a-Si:H) as the semiconductor and silicon oxide and/or silicon nitride as the gate insulator. Recent developments in materials have led to the exploration of organic oligomers such as hexathiophene and its derivatives, and organic molecules such as pentacene (G. Horowitz, D. Fichou, X. Peng, Z. Xu, F. Garnier, Solid State Commun. Volume 72, pg. 381, 1989; F. Garnier, G. Horowitz, D. Fichou, U.S. Pat. No. 5,347,144) as potential replacements for amorphous silicon as the semiconductor in thin-film field-effect transistors.
Recently, organic TFT comprising relatively high dielectric constant gate dielectrics were described (C. D. Dimitrakopoulos, B. K. Furman, S. Purushothaman, D. A. Neumayer, R. B. Laibowitz, P. R. Duncombe, (Docket No. YO997-057) filed on the same day herewith and incorporated herein by reference) which have the advantage of relatively low voltage operation and performance comparable to that of amorphous silicon TFT.
The fabrication of metal oxide films by chemical solution processing has been described recently, especially the case in which metal alkoxyalkoxide solutions are employed (D. A. Neumayer, P. R. Duncombe (Docket No. YO997-069) filed on Mar. 10, 1997 and incorporated herein by reference).
The electrical characteristics of TFT's having pentacene as the semiconductor, a heavily doped Si-wafer as the gate electrode, thermally grown SiO.sub.2 on the surface of the Si-wafer as the gate insulator, and Au source and drain electrodes, are adequately modeled by standard field effect transistor equations (S. M. Sze "Physics of Semiconductor Devices", Wiley, New York, 1981, pg. 442), as shown previously (G. Horowitz, D. Fichou, X. Peng, Z. Xu, F. Garnier, Solid State Commun. Volume 72, pg. 381, 1989; C. D. Dimitrakopoulos, A. R. Brown, A. Pomp, J. Appl. Phys. Volume 80, pg. 2501, 1996). The pentacene used in these devices behaves as a p-type semiconductor. When the gate electrode is biased negatively with respect to the grounded source electrode, pentacene-based TFT's operate in the accumulation mode and the accumulated carriers are holes. At low V.sub.D, I.sub.D increases linearly with V.sub.D (linear region) and is approximately given by the equation: ##EQU1## where L is the channel length, W is the channel width, C.sub.i is the capacitance per unit area of the insulating layer, and V.sub.T is a threshold voltage. The field effect mobility, .mu. can be calculated in the linear region from the transconductance: ##EQU2## by plotting I.sub.D vs. V.sub.G at a constant low V.sub.D and equating the value of the slope of this plot to g.sub.m.
When the drain electrode is more negatively biased than the gate electrode (i.e. -V.sub.D .gtoreq.-V.sub.G), with the source electrode being grounded (i.e. V.sub.S =0), the current flowing between source and drain electrodes (I.sub.D) tends to saturate (does not increase any further) due to the pinch-off in the accumulation layer (saturation region), and is modeled by the equation: ##EQU3##
The field effect mobility can be calculated from the slope of the .sqroot..vertline.I.sub.D .vertline. vs. V.sub.G plot.